Aquifer Mapping and Characterization in the Complex Transition Zone of Ijebu Ode, Southwestern Nigeria
Abstract: Vertical electrical soundings (VES) and geophysical logs were employed to map and characterize the aquifer units in the Northwestern zone of Ijebu Ode, Southwestern Nigeria with a view to appraise the groundwater potential of the area. Sixteen Schlumberger soundings (VES) having maximum current electrode separation of 900 m were acquired and interpreted through partial curve matching and computer iteration. Gamma ray and resistivity logs acquired in a drilled hole were interpreted for aquifer characteristics. All but one sounding (VES 9), indicated signatures that are diagnostic of poor hydrogeological characteristics. Four layers were interpreted within 80 m depth. The first layer composed of topsoil (dry clay) which ranges in thickness from 0.8 to 1.5 m. Sandy clay (53 – 1895 Ωm) with varying thickness (0.8 – 34.5 m) constitute the second layer. Thick sand (2.4 – 55.3 m) having high resistivity (1208 – 7350 Ωm) make up the third layer. Resistive basement (3155 – 39529 Ωm) occurring at depth of 3 – 63 m constitute the fourth layer. The low resistivity sand (1023 Ωm) located beneath VES 9 was identified to be the saturated aquifer. The saturated aquifer has 8 – 10% clay content and 40% porosity. The aquifer is 100% saturated with fresh water having resistivity of 122 Ωm and TDS value of 53 ppm. This study showed that the area was of low groundwater potential and highlights the significance of combined surface and subsurface geophysical investigations for groundwater in area where groundwater occurrence is erratic.